We report systematic, measurement-induced irreversibility in quantum circuits on superconducting processors (ibm_kingston and ibm_marrakesh). Using molecular hydrogen (H2, STO-3G basis) as a minimal quantum-chemical model, we prepare ground states at five bond distances R = 0.735–3.0 Å spanning equilibrium to the dissociation limit, and implement time-reversal protocols (U → Mmid-circuit measurements → U†) in both single-qubit and two-qubit entangled configurations. The single-qubit protocol yields a fidelity deficit ΔF that saturates at M = 1 in quantitative agreement with the standard quantum-mechanical projection postulate (ΔF = sin2θ/2, R2 = 0.9997), serving as a control. The two-qubit entangled protocol yields a ΔF that grows linearly with M, with a per-measurement slope a ≈ 0.012 ± 0.002 that is independent of R (CV = 16.2% across 10 data points on two backends). The single- versus two-qubit contrast directly demonstrates that entanglement is a necessary condition for M-dependent accumulation of measurement-induced irreversibility. Independent generic-circuit experiments confirm that the effect is independent of free-evolution time T and reproduces across backends. All tested standard noise mechanisms fail to account simultaneously for these observations; the theoretical implications are discussed. Running title: Measurement-Induced Irreversibility in H2 Circuits Keywords: mid-circuit measurement; measurement-induced irreversibility; H2 dissociation; entanglement; variational quantum eigensolver; superconducting quantum processor
We report systematic, measurement-induced irreversibility in quantum circuits on superconducting processors (ibm_kingston and ibm_marrakesh). Using molecular hydrogen (H2, STO-3G basis) as a minimal quantum-chemical model, we prepare ground states at five bond distances R = 0.735–3.0 Å spanning equilibrium to the dissociation limit, and implement time-reversal protocols (U → Mmid-circuit measurements → U†) in both single-qubit and two-qubit entangled configurations. The single-qubit protocol yields a fidelity deficit ΔF that saturates at M = 1 in quantitative agreement with the standard quantum-mechanical projection postulate (ΔF = sin2θ/2, R2 = 0.9997), serving as a control. The two-qubit entangled protocol yields a ΔF that grows linearly with M, with a per-measurement slope a ≈ 0.012 ± 0.002 that is independent of R (CV = 16.2% across 10 data points on two backends). The single- versus two-qubit contrast directly demonstrates that entanglement is a necessary condition for M-dependent accumulation of measurement-induced irreversibility. Independent generic-circuit experiments confirm that the effect is independent of free-evolution time T and reproduces across backends. All tested standard noise mechanisms fail to account simultaneously for these observations; the theoretical implications are discussed. Running title: Measurement-Induced Irreversibility in H2 Circuits Keywords: mid-circuit measurement; measurement-induced irreversibility; H2 dissociation; entanglement; variational quantum eigensolver; superconducting quantum processor
ABSTRACT We report a systematic, measurement-induced irreversibility in superconducting quantum circuits that is not accounted for by any tested conventional noise model. Using IBM Quantum processors (ibm_marrakesh and ibm_kingston), we implement a time-reversal protocol in which mid-circuit measurements are inserted between a unitary U and its inverse U†. The fidelity deficit ΔF = F(dummy) − F(meas) grows linearly with measurement count M, reaching ΔF = 0.235 (22.2σ) and ΔF = 0.164 (24.0σ) at M = 8 on the two processors. Three independent controls rule out conventional noise sources: (1) T-independence of the spectator-qubit phase shift (dA ≈ +0.10 at T = 40 and 80 μs) eliminates crosstalk and leakage; (2) a non-Markovian symmetry test (spread = 1.5σ) eliminates decoherence asymmetry; and (3) cross-backend reproducibility of the linear ΔF(M) scaling eliminates hardware-specific measurement noise. The three observations share a common structural feature: the irreversibility is tied to discrete measurement events rather than free-evolution time, accumulates proportionally to their count, and reproduces across independent hardware. We report these observations as experimental evidence consistent with the interpretation that the imaginary unit i in the Schrödinger equation reflects a physically real internal structure of time, and that quantum measurement drives an irreversible projection of that structure onto the real axis. This interpretation is developed from first principles in Sec. II and connected to an existing theoretical framework [13] in Sec. VI. Keywordsquantum computing, mid-circuit measurement, complex time, irreversibility, IBM Quantum, time-reversal protocol
We investigate measurement-induced effects on trans-horizon correlations in dynamic quantum circuits executed on IBM's 156-qubit superconducting processor (ibm_fez, Heron r2 architecture). We engineer a 40-qubit spin chain with a spatially-varying XX + YY coupling profile that creates an analog horizon at position xh = 20, where coupling strength reaches a minimum. We define a correlation contrast metric C = ⟨σxiσxj⟩horizon − ⟨σxσx⟩exterior and compare three conditions: (i) unitary Trotter evolution (baseline), (ii) mid-circuit measurement with reset on qubits near the horizon, and (iii) control measurement on distant qubits. Each circuit was executed with 16,384 shots. The near-horizon measurement produces a (56.5 ± 1.9)% reduction in correlation contrast (95% CI from n = 5000 bootstrap resamples), while the control preserves (87.8 ± 2.1)% of baseline. Two ablation experiments constrain interpretation: removing the reset operation yields only a (2.6 ± 1.2) percentage point change, indicating that projective measurement—not state reset—drives the effect. Crucially, moving the measured subset 4 links away from the horizon collapses the reduction to (6.5 ± 2.3)%, yielding an 8.2 ± 1.4 ratio that demonstrates strong spatial localization. We additionally performed a flat-spacetime control with uniform coupling J(x) = Jmax on ibm_torino (Heron r1, 133 qubits), obtaining a baseline contrast that collapses to a small residual near zero (C = − 0.015, 95% CI [− 0.029, − 0.002]), confirming that the spatial structure arises from the engineered coupling profile rather than generic measurement artifacts. These results establish that mid-circuit measurement near an engineered coupling minimum preferentially disrupts correlations across that boundary, a signature qualitatively consistent with information-theoretic models of horizon physics.
Abstract We investigate measurement-induced effects on trans-horizon correlations in dynamic quantum circuits executed on IBM's 156-qubit superconducting processor (ibm_fez, Heron r2 architecture). We engineer a 40-qubit spin chain with a spatially-varying XX + YY coupling profile that creates an analog horizon at position x h = 20, where coupling strength reaches a minimum. We define a correlation contrast metric C = ⟨σ x iσ x j⟩ horizon − ⟨σ x σ x ⟩ exterior and compare three conditions: (i) unitary Trotter evolution (baseline), (ii) mid-circuit measurement with reset on qubits near the horizon, and (iii) control measurement on distant qubits. Each circuit was executed with 16,384 shots. The near-horizon measurement produces a (56.5 ± 1.9)% reduction in correlation contrast (95% CI from n = 5000 bootstrap resamples), while the control preserves (87.8 ± 2.1)% of baseline. Two ablation experiments constrain interpretation: removing the reset operation yields only a (2.6 ± 1.2) percentage point change, indicating that projective measurement—not state reset—drives the effect. Crucially, moving the measured subset 4 links away from the horizon collapses the reduction to (6.5 ± 2.3)%, yielding an 8.2 ± 1.4 ratio that demonstrates strong spatial localization. We additionally performed a flat-spacetime control with uniform coupling J(x) = J max on ibm_torino (Heron r1, 133 qubits), obtaining a baseline contrast that collapses to a small residual near zero (C = − 0.015, 95% CI [− 0.029, − 0.002]), confirming that the spatial structure arises from the engineered coupling profile rather than generic measurement artifacts. These results establish that mid-circuit measurement near an engineered coupling minimum preferentially disrupts correlations across that boundary, a signature qualitatively consistent with information-theoretic models of horizon physics.
Abstract Following the Governed Fault-Tolerant Quantum Computation of molecular ground states in the companion work [1]—H₂, LiH, H₂O, and BeH₂ at equilibrium and its strong-multireference transition state, all committed inside chemical accuracy (the BeH₂ equilibrium commit, at ΔE = +0.595 µHa, sitting 7.7× inside wavenumber accuracy—1 cm⁻¹ ≈ 0.00456 mHa, the precision regime of high-resolution molecular spectroscopy—the only QPU chemistry commit on record inside that band)—we extend Governed Fault-Tolerant Quantum Computing (GFTQC), executed under the governance of Seed IQ (the Adaptive Multiagent Autonomous Control platform of AIX Global), from chemical accuracy on those molecules to exact reproduction of a full-configuration-interaction benchmark: the linear hydrogen chain, the community's canonical test of strong electron correlation, established by Motta et al. [2] as a single full-configuration-interaction (FCI) equation of state of H₁₀ onto which fifteen-plus independent classical many-body methods were brought into agreement, and the accepted target any exact solver must hit.On Motta's own basis (STO-6G) the committed energy of H₁₀ reproduces his published FCI reference exactly, to twelve decimal places where his published precision is eight, at every point of the dissociation curve—at R = 1.8 bohr −5.424385376333 Ha against the published −5.42438538—each value internally certified exact. We then extend the benchmark in the two directions that turn a model system into chemistry: in length, carrying the exact energy from H₁₀ out to a chain of H₁₀₀; and in basis, computing it across four Gaussian bases from minimal to triple-zeta—STO-6G, 6-31G, cc-pVDZ, cc-pVTZ, the correlation-consistent sets of production electronic-structure theory—recovering progressively more correlation as the basis is enriched, each energy committed exact. Throughout, single-reference coupled cluster is exact near equilibrium but ceases to represent the state as the chain stretches and the dominant-determinant weight |c_HF|² collapses, whereas the governed commit stays exact at every length and every basis.These extensions lie beyond exact classical reach—the configuration space exceeds 10⁵⁸ determinants for H₁₀₀, unstorable on and unsolvable by any high-performance computer—and beyond any reported quantum method; to our knowledge the exact hydrogen-chain energies beyond Motta's classical H₁₀, in production bases and out to H₁₀₀, have not been computed before by any method. No error mitigation, post-selection, or classical fitting is used; every energy is read from the executed fault-tolerant state over qubit-wise-commuting measurement lanes on IBM Boston (Heron r3), accessed by AIX Global, and internally certified exact by a proprietary mechanism not disclosed here. This is the quantum value of fault tolerance made concrete on a strongly-correlated system: the exact answer, delivered where classical computing cannot store the problem and single-reference methods cannot represent the state at all. These results establish a new state of the art for exact electronic-structure computation on hardware: the first exact hydrogen-chain energies beyond H₁₀, by any method, classical or quantum.
Validating quantum processors at scale increasingly relies on approximate classical simulation, which adds truncation error to the comparison between ideal and observed behavior. We describe a 15-qubit dynamic-circuit benchmark that stays within exact state-vector simulation, so the ideal reference value is computed without approximation. The circuit scrambles a 6-qubit system, couples it to a 5-qubit engineered environment, reads out four weakly coupled probe qubits mid-circuit, and then applies the inverse of the scrambling and system–environment coupling to restore the system's coherence. Exact summation over the 16 probe outcomes gives an ideal recovery fidelity F_max = 0.9889 on the 11-qubit system–environment register; the shortfall from unity reflects information carried away by the probes, whose coupling is not inverted and which are discarded after readout. We demonstrate the pipeline with a single 400-shot run on the Quantinuum H2-1 emulator (quantinuum.sim.h2-1e) via Azure Quantum. A local surrogate built from three published H2-1 headline error rates predicted F_exp = 0.8353 ± 0.0054; the emulator returned a readout-mitigated F_rec = 0.7836 ± 0.0212, 2.36 nominal standard errors lower. Because the surrogate's quoted uncertainty reflects sampling error only, we interpret the shortfall as evidence of surrogate-model mismatch—expected, given the emulator's richer noise model—rather than as a statistically calibrated anomaly. No physical-hardware data are reported. Changes in v50: Retitled (formerly "A 15-Qubit Decoherence-Reversal Protocol for Benchmarking Mid-Circuit Measurements and Environmental Imprinting"). Corrected the echo formulation (v49 Eq. 4 was identically 1) and the reported raw frequency (77.5%, not 76.0%). Result reframed as a nominal surrogate-model mismatch on the H2-1 emulator rather than anomalous decoherence; SWAP-routing explanation withdrawn. References corrected; limitations and data/code availability added. Analysis code unchanged from v48. Code and data: https://github.com/dkahng/15-qubit-mcm-protocol
Abstract Dynamic circuits with mid-circuit measurement (MCM) and classical feed-forward are essential building blocks for quantum error correction and adaptive quantum protocols. However, in superconducting quantum processors the readout process can induce coherent crosstalk on non-measured (spectator) qubits through the readout resonator, residual photons, and other hardware pathways, leading to deterministic phase kicks (effective Z rotations) that accumulate with the number of MCM events. In this work we present a minimal, reproducible two-qubit Ramsey diagnostic to quantify the spectator-induced phase shift as a function of the number of inserted mid-circuit measurements M, and we demonstrate that a simple virtual-Z (frame) compensation proportional to M can mitigate the resulting phase error on real hardware. In a high-SNR dataset archived on Zenodo (T = 40 μs, 4096 shots per circuit), the effective phase difference Δθeff(M) = θMEAS(M) - θDELAY(M) shifts monotonically in the negative direction as M increases, reaching Δθeff (M=8) = -1.944 ± 0.033 rad. While the contrast r decreases mildly with M, it remains relatively high (r ≈ 0.84 at M = 8), consistent with the effect appearing predominantly as a coherent phase rotation. To mitigate the error, we insert a virtual-Z rotation on the target qubit of the form Rz (comp × M) in the MEAS condition and scan the compensation coefficient comp (rad per measurement). In a MAP dataset (ibm_marrakesh, shots = 512, repeats = 1, M ∈ {0,1,2,4,8}, comp ∈ {-0.02,-0.01,0,+0.01,+0.02}), the RMS of the baseline-referenced phase error Δθeff(M) for M>0 is minimized at comp = -0.01, decreasing from 0.1393 rad (baseline) to 0.0601 rad. Finally, we implement a low-cost calibration probe (M = {0,8}) scanning five compensation settings comp ∈ {-0.02,-0.01,0,+0.01,+0.02} (1024 shots per circuit, repeats = 2) and estimate a session-dependent coefficient compopt ≈ -0.013 rad/measurement from a linear fit of Δθeff(M=8) versus comp. A same-session production sweep using comp = -0.01453 rad/measurement (close to compopt) shows a reduced residual phase error, supporting a practical probe → production workflow for dynamic-circuit execution. All analysis scripts and summary data are made publicly available to support independent reproduction of the reported phase metrics and figures.